Abstract

1.54 μm Si-anode organic light emitting devices with Er(DBM)3phen: Bphen and Bphen/Bphen:Cs2CO3 as the emissive and electron transport layers (the devices are referred to as the Bphen-based devices) have been investigated. In comparison with the AlQ-based devices with the same structure but with AlQ:Er(DBM)3Phen and AlQ as the emissive and electron transport layers, the maximum EL intensity and maximum power efficiency from the Bphen-based devices increase by a factor of 3 and 2.2, respectively. The optimized p-Si anode resistivity of the Bphen-based device of 10 Ω·cm is significantly lower than that of the AlQ-based device. The NIR EL improvement can be attributed to the energy transfer from Bphen to the Er complex and equilibrium of electron injection from the Sm/Au cathode and hole injection from the p-Si anode at a higher level.

]. The NIR luminescence of ~1.54 μm from erbium-containing materials has been widely investigated for a wide range of applications in optical telecommunications and Si photonics, because this wavelength lies in the minimum-loss transmission window of silica-based fibers. Since Gillin reported Silicon-based OLEDs using ErQ as emitting layers, many efforts have been done to obtain ~1.54 μm EL and Er3+ complexes have become a very active field of research for the 1.54-μm light-emitting devices [4

], we have fabricated Si-based OLEDs using erbium (III) 2,4-pentanedionate [Er(acac)3] as the dopant, tris-(8-hydroxyquinoline) aluminium [AlQ] as the emissive host and the electron transporting layer for the 1.54 μm light emission. However, the EL intensity of the devices is still not strong. 4,7-diphenyl-1,10-phenanthroline [Bphen] has been used as good electron transporting/hole blocking material to enhance the EL properties in visible light-emitting devices [9

]. In this report, we use Tris (dibenzoylmethane) mono (phenanthroline) erbium(III) [Er(DBM)3phen] as dopant, Bphen as emissive host and electron transport layer (ETL) to further enhance the Er3+ electroluminescence. The increasing NIR emission from Er3+ can be achieved due to the large overlap between Bphen and Er(DBM)3phen which can lead to an energy transfer process. Furthermore, we demonstrate that since Bphen, a more efficient electron transport material is used instead of AlQ to improve the electron injection, then hole injection of the p-Si anode can be properly enhanced by suitably reducing the resistivity of p-Si and the power efficiency is efficiently improved.

2. Experimental

The Erbium complex, Er(DBM)3Phen was prepared by the method reported before [12

]. P-type (100) silicon wafers (GRINM, China) with a resistivity of 1-40 Ω·cm were used as the anodes and substrates. To improve the device performance, 2 nm SiO2 layers were grown on their front sides after the Si wafers were routinely cleaned [13

]. And then, Ohm contacts were formed on their backsides after evaporating Al. They were then transferred into an evaporation chamber with a base pressure of 4 × 10−6 Torr. A 60-nm-thick N, N’-bis -(1-naphthl) -diphenyl-1,1’-biphenyl-4, 4’-diamine [NPB] layer is used as the hole transport layer. The 20-nm-thick emitter layer is Er(DBM)3phen, which is doped into the host material Bphen using co-evaporation method. The ETL Bphen and another 15 nm Cs-doped Bphen are using as by as the electron injection layer and a part of the ETL respectively. A stacked Sm/Au bilayer is used as the cathode, and the optimized thickness of Sm is 15 nm by which the light emission would be stable. Thus, the resulting device we explored is p-Si substrate/ SiO2 1.5 nm/ NPB 60 nm/ Bphen:Er(DBM)3phen 20 nm/ Bphen 25 nm/ Bphen: Cs2CO3 ‖)(∏{∼mass ratio of 1:1| 15 nm/Sm 15 nm/Au 15 nm. In order to compare with the previous devices, AlQ is also used as emission layer and ETL respectively. As shown in Fig. 1

The near infrared electroluminescence (EL) was driven by square pulses with a frequency of 22 Hz and a duty cycle of 1: 1, and measured by a ADC 403L (Applied Detector, Fresno, CA) liquid-nitrogen cooled Ge detector at room temperature. Power efficiency was determined by source meter (Keithley 2400) with a calibrated InGaAs photodiode Module (Hamamatsu G6121). Photoluminescence (PL) of Bphen thin film was performed by using a microzone confocal Raman spectrometer (LabRam HR 800) and the absorption of Er(DBM)3Phen was examined using a UV-Vis spectrophotometer (Lambda 35).

The devices were not encapsulated and all the measurements were carried out in atmosphere at room temperature.

3. Results and discussion

The typical top-emission electroluminescence spectra for the Si-based OLEDs are shown in Fig. 2

Fig. 2 EL spectra measured at the current density of 200 mA/cm2 for Bphen-based device with 10 Ω·cm p-Si anode, the AlQ-Bphen device with 10 Ω·cm p-Si anode and the AlQ-based devices with 40 Ω·cm p-Si anode. The inset shows the PL spectrum of Bphen film (λex=325 nm) and absorption spectrum of Er(DBM)3Phen.

. It can be seen that all the devices show the typical 1.54 μm peak which is the characteristic emission of Er3+ ions, attributing to the transitions, 4I13/2 ~4I15/2. At the same current density of 200 mA/cm2, the Bphen-based device exhibits near four and six times higher EL intensity at 1.54 μm than that of the AlQ-Bphen and AlQ-based devices respectively.

To study the effect of electric resistivities of p-Si anodes to the NIR EL, we have fabricated Bphen-based devices each with a 40, 20, 10, 1 and 0.08 Ω·cm p-Si anode. Figure 3

Fig. 3 The NIR power of the Bphen-based and AlQ-based devices versus resistivity of the p-Si anodes.

summarized the maximum emission intensity of these devices. As it can be observed, the power efficiency of Bphen-based devices strongly depends on electrical resistivity of the passivated p-Si anode, and the optimum electrical resistivity for the passivated p-Si anode is 10 Ω·cm. However, to AlQ-based devices, the best electrical resistivity we can obtain for the passivated p-Si anode is 40 Ω·cm. This result implies that when electron current is enhanced by using CsPh instead of AlQ, hole current can be enhanced by simply reducing the resistivity of the passivated p-Si anode to 10 Ω·cm for matching electron current.

The NIR irradiation-voltage and current density-voltage curves for Bphen-based device with 10 Ω·cm p-Si anode are plotted in Fig. 4

. The NIR irradiation curve of Bphen-based device shows an apparently sublinear increase with current density up to the maximum current density of 700 mA/cm2 that we used and the maximum NIR power of 0.93 μW/cm2 is acquired at the current density of 635 mA/cm2 (12.5 V). Besides, it is worth noting that the NIR emission turn-on voltage of the Bphen-based devices is ~7 V which is farther lower than that of reported ErQ-based device [4

]. The maximum EL intensity and maximum power efficiency of the Bphen-based device with 10 Ω·cm p-Si anode is increased by a factor of 3 and 2.2, comparing to the AlQ-based device with 40 Ω·cm p-Si anode.

The improvement of the power efficiency should be ascribed to two aspects as follows. Firstly, comparing with AlQ, Bphen has a relative higher electron injection capability [14

]. Thus in the AlQ-based device, to keep a carrier balance, a relatively lower hole injection from higher-resistivity p-Si anode is needed to match the electron injection. Correspondingly, the resistivity of p-Si anode should be no less than 40 Ω·cm. Whereas in Bphen-based device, the hole injection from the p-Si anode should be enhanced by decreasing its electrical resistivity to keep a carrier balance. Consequently, the maximum power efficiency from the Bphen-based device is larger than that of the AlQ-based device. Secondly, in Bphen-based device, it can be seen a large overlap between PL spectrum of Bphen and absorption spectrum of the ligands (DBM and phen) as plotted in the inset of Fig. 2. This implies an efficient Föster energy transfer process from Bphen to Er(DBM)3phen which may increasing NIR emission from Er3+ [15

], resulting in an inefficient EL at 1.54 μm. Thus, in Bphen-based device, the EL of Er(DBM)3phen can be obtained in two ways. Except directly from trapped carriers in emissive layer, the EL can be achieved from the energy transfer process [18

]. Since holes are injected from p-Si anode and transported into the Er doped Bphen emissive layer, the energy from Bphen can be effectively transferred to the Er complex. Then ligands DBM and phen are able to transfer the absorbed energy to the central metal Er3+ ion according to the antenna effect. Further investigation for the EL mechanism is still ongoing.

3. Conclusion

In summary, we demonstrate an improved 1.54 μm NIR emission from Si-anode OLED using Er(DBM)3Phen doped Bphen as emissive layer and Bphen as ETL. We conclude that the enhancement of the NIR EL attribute to both the higher electron transport ability of Bphen and the intersystem energy transfer between Er complex and Bphen. The results also indicate that a suitable p-Si anode for the devices plays an important role for the NIR emission because of the balance between electron- and hole-injection can optimize the NIR light-emitting efficiency.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (Grant Nos. 50732001, 10674012, 10874001, and 60877022), the National 973 Project (Grant No. 2007CB613402) China Postdoctoral Science Foundation.

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